This change comprises a few tiny changes wrt Member:
1) Move member policies to a separate file so that some of them
(CheckingPolicy) can be reused by Persistent;
2) SFINAE out incompatible pointers from heterogeneous ctor/asgnmt;
3) Rename kMemberSentinel to kSentinelPointer.

Bug: chromium:1056170
Change-Id: I4482998e6ba61005a5d0861dcae9fab2aa43702c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2139587Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67061}